In recent years, nondestructive sensing techniques using an electromagnetic wave o f millimeter-wave to terahertz (THz) frequencies (30 GHz to 30 THz) have been under development. Examples of techniques now under development in “application fields of the electromagnetic wave of such a frequency band include imaging a technique using a safe fluoroscopic apparatus alternative to an X-ray fluoroscope, a spectral technique for obtaining an absorption spectrum or complex dielectric constant of a material to inspect (or examine) the bonding state therein, a technique of analyzing biomolecules, and a technique for estimating a carrier concentration or mobility.
As a sensing system using a terahertz electromagnetic wave, Japanese Patent application Laid-open No. H10-104171 discloses a method of preparing a photoconductive element in which antennas also serving as electrodes are provided on a photoconductive film formed on a substrate and irradiating the photoconductive element with an extremely-short pulse laser light to generate and detect a terahertz electromagnetic wave. Since absorption lines of various materials are present in the terahertz range, it is becoming more important to apply a terahertz electromagnetic wave to sensing techniques such as analyses of plastics, composite materials, or the like, water content analysis, and biomolecule analysis. By moving an inspected (or examined) material two-dimensionally, it is also possible to obtain a two-dimensional image through imaging based on, for example, a difference in absorption coefficient of an electromagnetic wave.
In such a case, there have hitherto been such problems that optical axis alignment is required because a spatial imaging system is used, and that it is hard to increase the sensitivity when measuring a powder material or liquid material. Therefore, in recent years, there has been proposed a method of locating an inspected material on a THz-wave reflection prism and performing sensing utilizing a THz evanescent wave generated by total reflection on an upper surface of the prism (see Extended Abstracts of The 51st Meeting of The Japan Society of Applied Physics and Related Societies, 28p-YF-7). In this case, since an absorption spectrum or the like is measured based on the interaction between the evanescent wave and the inspected material, the measurement is available regardless of any form the object takes, such as liquid, power, or the like.
However, the method using the prism has limitations in reducing the beam diameter of a propagating terahertz-electromagnetic wave, so that there can typically be obtained only a maximum spatial resolution of about 100 μm which corresponds to the order of a wavelength of the terahertz electromagnetic wave. Therefore, when a two-dimensional image of an inspected object is to be observed or inspected objects are to be arranged in a two-dimensional array form, higher resolution measurement is required. Further, the use of the prism requires expensive optical parts having low-losses in the THz band, and imposes limitations in size reduction of the entire optical system. Moreover, since the optical system is liable to be affected by moisture in air, it is necessary to locate the entire optical system in a nitrogen atmosphere, thus hindering the cost reduction.